1. Technical Field
The present disclosure relates to low power wideband local oscillators.
2. Background Information
Wireless transceivers generally use one or more local oscillator circuits to generate signals referred to as Local Oscillator (LO) signals. These LO output signals are used by other circuitry in the transceiver either to upconvert a baseband signal in frequency for transmitting, or to downconvert an RF signal in frequency during receiving. The LO output signals often must involve quadrature signals. A divider circuit is therefore traditionally used to divide down a higher frequency signal by two and thereby to output both an In-Phase (I) LO output signal and a Quadrature-Phase (Q) LO output signal, where the I and Q LO output signals are both at half the frequency of the signal supplied to the divider but where the I and Q signals are ninety degrees out of phase with respect to one another. In a narrowband transceiver, an injection-locked type divider can be used for this purpose of generating the I and Q signals. As compared to a differential latch type divider, the injection-locked divider may exhibit lower power consumption and lower phase noise. The injection-locked divider however has a locking range due to the natural oscillating frequency of a part or parts of the circuit. A reference signal is supplied to a Phase-Locked Loop (PLL) such that the PLL outputs a signal of twice the frequency of the desired quadrature signals. The PLL output signal is tunable. The PLL output signal is made to inject energy into the injection-locked oscillator in such a way that a resonator of the injection-locked oscillator oscillates at the frequency or at a frequency fraction of the input PLL signal. For example, the ILO divider may oscillate at a frequency that is ½, or ⅓, or ¼, etc., of the frequency of the PLL output signal. The result is that the injection locked oscillator outputs I and Q signals of the desired output frequency, where this frequency is tunable by controlling the PLL appropriately.
Using an injection locked oscillator to perform frequency division works well in many applications, but using the injection locked oscillator has a drawback in that the oscillator generally can only lock to signals in a narrow frequency band. Locking of the injection locked oscillator is limited in this way because the oscillator has a high Q resonator. Where the device that uses the I and Q LO output signals is the receiver and/or transmitter within a wideband transceiver, such an injection locked oscillator cannot lock at all the PLL signal frequencies required. This is especially true when a large population of devices is considered, over temperature, and process and supply variations that the transceivers will experience. There are techniques for making an injection locked oscillator type divider more adaptable, but these techniques degrade divider performance. Accordingly, in wideband transceivers a differential latch type divider is generally employed to generate the LO output signals. However, a differential latch divider circuit has a higher power consumption and phase noise than an injection locked oscillator even though it can divide down the PLL signal over a wider frequency range.